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We have been studying for long time topics in macromolecular
chemistry and self-assembly of nanostructures that have become of wide
interest in recent years. Highly stable nanostructured materials with
macromolecules obtained by inclusion polymerisation or self-assembly have
wide implications in macromolecular stereochemistry and in the dynamics of
polymeric chains isolated or confined to restricted spaces. The topology and
the interactions at the interfaces could be understood by targeted
experiments of 1D and 2D advanced Solid State Nuclear Magnetic Resonance.
Nuclear Magnetic Resonance techniques.
Novel nanoporous and mesoporous materials (0.5 – 3.5 nm cross-section)
forming regular nanochannels are the main architectures that we could
fabricate. Some molecular crystals contain receptors for specific
interactions with gas atoms, molecules and macromolecules, and are stable as
organic zeolites, even upon removal of the guests. The open crystalline
nanochannels absorb volatile molecules from the gas phase in the unique
aromatic environment.
These compounds enabled the formation of novel crystalline materials for
opto-electronic applications and used for storage and purification of gases.
The purely organic crystals could be loaded with methane and carbon dioxide,
even at low pressure. The unprecedented result was obtained by exploiting
weak interactions to stabilize a supramolecular porous architecture and by
the participation of the gases into the network. Direct evidence of the
host-guest interactions was provided by the rare NMR observation of gases
trapped into crystals. The dramatic magnetic susceptibility shifts of the
stored gases indicate that these sit at close contacts with the surrounding
aromatic environment.
Hyperpolarized xenon NMR is operative in our laboratories for the study of
porous materials and polymers. This laser-assisted technique reaches extreme
sensitivity and allowed us to determine the shape and symmetry of pores and
channels of comparable dimensions to xenon radius.
Narrow nanochannels of porous crystals can be explored by xenon atoms and
cannot bypass each other realizing a particular case of one-dimensional
diffusion, called single-file diffusion. Laser-polarized 129Xe NMR in the
Continuous-Flow set-up made it possible to monitor diffusion over a
time-scale of tens of seconds, often inaccessible by conventional NMR
experiments. The xenon gas atoms squeezed into the crystal at various
pressure and temperatures could modulate the signal anisotropy in such a way
to reveal xenon-xenon and xenon-walls interactions.